The invention relates to a device for transmitting and receiving circularly polarized microwaves.
Devices of this kind are routinely used in telecommunications systems. They are usually intended to transmit high powers and receive low powers. This applies to telecommunications systems in which the signals are relayed by geostationary satellites, for example.
In such devices, the transmit frequencies and the receive frequencies are different to prevent the transmit signals interfering with the receive signals. It is also necessary to provide filter means so that the required frequency can be received or transmitted on each channel and the frequency of the other channel eliminated. The signals must be particularly carefully separated when transmission and reception are simultaneous.
Such systems usually include a waveguide source and a duplexer with a high rejectivity in the transmit and receive bands, respectively. They are therefore bulky, which may not be suitable in all applications, in particular in telecommunications systems terminals in which each user must be provided with a transceiver.
In particular, microwave transceiver devices are routinely used for domestic and professional applications in satellite telecommunications systems.
Telecommunications systems of this type are being developed for "multimedia" applications, for example. Such systems include a constellation of satellites in low Earth orbit, at an altitude of 800 km to 1500 km, or medium Earth orbit, at an altitude of 6000 km to 12,000 km. These orbits are "low" and "medium" in comparison with the orbit of geostationary satellites at an altitude of 36,000 km. The role of the satellites is to provide communications between terrestrial users. The calls transmitted in this way are multimedia calls, i.e. they convey TV, audio and video signals, digital data of all kinds, programs, telephone signals or facsimile signals. Compared to calls relayed by geostationary satellites, the low altitude of the satellites reduces the distance the call has to travel and therefore the propagation time delays, which facilitates interactivity in such systems. Furthermore, with constellations of satellites it is possible to optimize the coverage, for example by concentrating resources in areas of high population density, whereas a geostationary orbit concentrates resources in areas near the equator.
A terrestrial user can communicate with a satellite only when the satellite is "in view"; this is generally for a period in the order of twenty minutes. It is therefore necessary for the user's antenna to be able to track the satellite as it moves overhead and for the user to be able to switch the call instantaneously to the next satellite, which is coming into their field of view at the same time as the present satellite is moving out of their field of view. Instantaneous switching is needed above all for interactive calls for which any interruption of service, however short, is unthinkable. To solve this problem, a transceiver device with two antennas is generally provided, one antenna moving to track the satellite with which the user is communicating and the other at the ready and pointing towards the beginning of the field of view of the next satellite.
Transceiver devices, and in particular their antennas, intended for such telecommunications systems must be particularly light and small in order to facilitate movement and installation on the roof of a building (in particular a private house) and thereby avoid an undesirable aesthetic effect.
Furthermore, it can be advantageous to associate the two transceiver devices with a common focusing lens. In this case, the two devices must coexist in a limited space, which reinforces the need for these devices to be small in size and light in weight.
Given the above conditions, it is hardly feasible to use a waveguide/duplexer configuration with a high rejectivity, which is heavy and bulky. A more compact technology is therefore used, that most widely employed being referred to as the "microstrip" technology. However, with this technology, solutions known in the art to the problem of isolating transmission and reception cause high losses which degrade link quality or make it necessary to increase the size of the antenna.
For example, document JP 10 022728 describes a circular polarization antenna using a technology of the above kind and used only to transmit or only to receive, and therefore for a single frequency band. Consequently, a transceiver device would have to comprise two antennas of the above type with a hybrid coupler because the same type of polarization is used for both frequency bands.
Document JP 06 140835 concerns a circular polarization antenna including a patch for transmitting and a patch for receiving. There is therefore one port for each frequency band, which means that the antenna is oversized.